PHYSICS OF NINETEENTH CENT. RADIATION. 521 



angles to each other, and the extremity of each arm bears a small 

 vertical piece of metallic foil, one surface of which is covered by lamp- 

 black, while the other is polished, and the several blackened and po- 

 lished surfaces are respectively turned one way. When radiant light or 

 heat falls upon this apparatus, its effect is that the little mill begins to 

 rotate in the same direction as if the radiant light or heat repelled the 

 blackened surfaces of the vanes. The velocity of rotation is proportional 

 to the intensity of the radiation which falls upon the instrument. It 

 has accordingly been proposed to employ the radiometer to measure 

 the intensity of light, instead of the ordinary photometrical methods 

 depending upon the estimation by the eye of equality of illumination. 



FIG. 240. FIG. 241. 



In following out the investigations which these discoveries sug- 

 gested, Mr. Crookes was led to certain experiments on the passage of 

 electricity in extremely attenuated media. On account of their rela- 

 tion to the other vacuum experiments, the results of those experiments 

 may be mentioned in this place rather than under the head of elec- 

 tricity, and also because the phenomena recall certain properties of 

 radiations. 



Let Fig. 240 represent a bulb exhausted by a Sprengel pump. 

 Platinum wires are sealed into the tube at the points P p' and N, the 

 latter terminating inside in a small flat surface. N is connected with 

 the negative pole of a Ruhmkorff's induction coil (Chap. XX.), and 

 then, according as p or p' is connected with the positive pole, the dis- 

 charge takes the form of a luminous track between p N, or between P'N ; 

 or both, P and P' may be simultaneously connected with the positive 

 pole, when the discharge will take the forms represented in the figure. 

 Fig. 241 shows an exactly similar bulb, in which, however, the ex- 

 haustion has been carried to the fuou5ou tn OI " an atmosphere. When 



